The fossa ovalis is an oval depression in the inferior part of the interatrial septum. It represents the foramen ovale of the fetus. The floor of the foramen ovale represents the fused wall of the septum primum of the fetus. Occasionally, fusion is incomplete and the superior part of the floor shows a communication through to the left atrium - persistent foramen ovale.
At the superior margin of the fossa ovalis is the limbus fossae ovalis.